Transparent ceramics are of great interest on account of their versatile optoelectronic properties and their potential for transparent armor related application. Foremost candidates for structural transparent ceramic armor materials are cubic aluminum oxynitride, known under the trade name AlON, cubic magnesium aluminate spinel and sapphire. The latter has a non-cubic crystal structure and is currently manufactured and used only in the form of single crystals, whereas cubic, isotropic AlON can be processed as polycrystalline materials. However, the raw material powders for AlON are expensive. Polycrystalline magnesium aluminate spinel (MgAl2O4) is currently the most promising optically transparent ceramics, exhibiting a unique combination of mechanical and optical properties. When sintering a composition containing spinel or oxide powders, the resulting sintered transparent spinel body displays an optically isotropic cubic structure, so that light scattering on the boundary surfaces of crystals is less critical. High transmittance body can be obtained by reducing the amount of pores and impurities in the polycrystalline structure.
Known methods for obtaining transparent polycrystalline sintered spinel set high demands on the raw materials, either employing organo-metal reagents or oxides of high purity and low particle size. Moreover, the known methods employ multi-stage, time and energy-consuming procedures, exhibiting complex temperature regimens. U.S. Pat. No. 4,543,346, for example, describes a transparent spinel body obtained from a mixtures of magnesium and aluminum isopropoxides by a multistage, complex process comprising keeping the body at high temperatures for twenty hours, the total process time going beyond 48 hours. U.S. Pat. No. 5,001,093 describes a spinel body obtained from magnesium and aluminum oxides of submicron size, with strictly limited impurities, some only on the level of several ppm, in a prolonged multistage process. Known methods, thus, provide transparent spinel bodies only by lengthy, complicated and costly procedures. It is therefore an object of this invention to provide a simpler method for manufacturing transparent polycrystalline sintered spinel ceramic.
It is another object of the invention to provide a method for producing transparent spinel bodies from oxide powders, without particular demands on their purity and particle size.
The size of the alumina and magnesia particles is of the order of <10 micron, preferably <5 micron.
It is still another object of the invention to provide a relatively less time-consuming and energy-consuming process for manufacturing transparent spinel.
It is a further object of the invention to provide a method capable of providing transparent spinel bodies in desired quantities and qualities within hours, from oxide powders of commercial purity.
It is a still further object of the invention to provide a rapid method for manufacturing a transparent polycrystalline sintered spinel bodies for use in optoelectronics and in armor-related applications.
It is also an object of the invention to provide relatively inexpensive magnesium aluminate spinel which is transparent essentially in the whole visible range.
Other objects and advantages of present invention will appear as description proceeds.